One way to optimize investment in optical fiber communications in order to cope with increase in information transmission demand in optical fiber communications is to employ a more efficient modulation scheme for transmitted information to increase the spectral efficiency.
To meet this direction, modulation schemes based on Quadrature Phase Shift Keying (QPSK), for example, have been developed for optical communication systems having larger capacities. In this case, information is encoded into four phase levels. Accordingly, 2 bits of binary signal per transmission symbol can be encoded.
Compared to a modulation scheme implemented using On-Off Keying (OOK), which encodes 1 bit per sample, the QPSK modulation technique can transmit twice the amount of information in the same required light spectrum bandwidth.
Techniques that further increase the communication capacity by improving the spectral efficacy per channel include a Quadrature Amplitude Modulation (QAM) technique, for example. In this case, a symbol is encoded into a phase level and an amplitude level and is constructed as a combination of multilevel modulations in quadrature phase. One example of the QAM scheme is disclosed in NPL 1. The modulation scheme in NPL 1 is a 16-QAM optical transmission and information is converted to 16 levels, that is, 4 bits of binary code per symbol in this case. This can double the light spectral efficiency compared to QPSK.
Further, NPL 2 discloses the use of 512-QAM. In this case, information is encoded into 512 levels, that is, 9 bits of binary code per symbol. The light spectral efficiency further increases compared to 16-QAM. The QAM scheme is therefore an efficient method of increasing the capacity of a communication line.
The QAM scheme can be implemented by using an optical In-phase/Quadrature (IQ) modulator. In the optical IQ modulator, two independent Mach-Zehnder devices are driven by electrical signals. They are called child Mach-Zehnder modulators (MZMs) (hereinafter abbreviated as “child MZMs”).
The two child MAMs modulate the phase and intensity of the same optical carrier. The optical phase of one of two child MZM outputs is relatively delayed by 90 degrees before being recoupled.
The phase delay between the outputs of the two child MZMs is referred to as quadrature angle. The quadrature angle is ideally 90 degrees in the 180-degree method.
While optical IQ modulators are used in the QAM modulation scheme in NPL 1 and NPL 2, optical IQ modulators are used in the QPSK modulation scheme as well. These IQ modulators provide an efficient proven method of implementing QAM modulation.
However, IQ modulators are known to exhibit direct current (DC) bias drift due to factors such as temperature changes or aging of devices.
There are three biases affected:    DC biases of two child MZMs, and    DC bias used for setting an angle to the quadrature angle.
This is already known for QPSK modulation and, for the QAM scheme if modulators that have the same structure are used.
When a drift occurs in DC bias, the drift causes an IQ modulator to operate inaccurately. This causes degradation of transmission signals from the IQ modulator. As a result, the quality of signals at a receiving unit degrades. Or, in the worst case, it becomes impossible to decode signals received at a receiving unit.
For OOK, phase shift keying (PSK) modulation, and QPSK, these problems have been solved by using an Auto Bias Control (ABC) circuit (hereinafter simply referred to as “ABC circuit”) which controls biases of modulators to compensate for variations in DC biases.
PTL 3 discloses a scheme that can be used for ABC that controls a 90-degree phase between outputs of Mach-Zehnder devices. This scheme is based on minimizing the Radio Frequency (RF) power spectrum of a modulated signal. Its basic principle is that interference between In-phase (I) and Quadrature (Q) data components enhances the RF power spectrum and therefore the quadrature angle can be controlled by minimizing the RF power spectrum.
By combining this scheme with a well-known method used for controlling the DC biases of child MZMs, the DC biases of an IQ modulator used for QPSK modulation can be controlled.
In PTL 1, the same principle as the principle in NPL 3 is used and, in addition, a dither frequency is added. The purpose is to control the quadrature angle by controlling a monitor signal for spectral components relating to the dither frequency.
Further, PTL 1 also describes in detail an ABC circuit based on dithering for controlling the DC biases of Mach-Zehnder devices. Like NPL 3, such a method is effective for QPSK. This can compensate for bias variations during operation and startup of a modulator used in QPSK.
PTL 2 indicates a configuration of an ABC circuit that is applicable to the 16-QAM modulation scheme. FIG. 1 illustrates the configuration (FIG. 1 is a diagram taken from FIG. 1 of PTL 2 without alterations).
Here, time-division switching is used to apply a low-frequency signal f0 dither to a child MZM 181 that performs modulation of an I-arm, apply the low-frequency signal f0 dither to a child MZM 18Q that performs modulation of a Q-arm, and apply the low-frequency signal f0 dither to a π/2-phase shift unit 19, an output from an optical monitor 21 of an IQ modulator 20 is synchronously detected at a synchronous detection unit 41 to increase the sensitivity of detection of each dither, and each DC bias is controlled in such a way that the dither signal becomes zero.
Further, PTL 3 discloses a configuration in which a phase shift unit provides a phase difference π/2 between an I-arm and a Q-arm, an optical receiver converts an optical output signal from a modulation unit to an electrical signal, a low-pass filter that has an cutoff frequency lower than a symbol frequency filters an output signal of the optical receiver, and a monitoring unit detects the power of an output signal of the filter, a phase-difference control unit controls a phase shift amount of the phase shift unit in such a way as to minimize the power of the filter output signal.
PTL 4 discloses a configuration in which when continuous light output from a light source is provided to a Differential Quadrature Phase Shift Keying (DQPSK) modulation unit including a phase modulation unit and a phase shift unit on each of arms A and B of a Mach-Zehnder interferometer to produce DQPSK signa light, a low-frequency pilot signal is superimposed only on a bias voltage to be supplied to either one of the phase modulation unit and the phase shift unit, whereby bias voltage control adapted to each of the phase modulation unit and the phase shift unit is performed based on the result of monitoring of signal light output from the DQPSK modulation unit.